The present invention relates to a staged combustion system in which the production of undesirable combustion product components is minimized over the engine operating regime and, more particularly, to a method and apparatus for actively controlling fuel flow to a mixer assembly having a pilot mixer with a primary fuel injector and secondary fuel injection ports.
Modern day emphasis on minimizing the production and discharge of gases that contribute to smog and to other undesirable environmental conditions, particularly those gases that are emitted from internal combustion engines, have led to different gas turbine engine combustor designs that have been developed in an effort to reduce the production and discharge of such undesirable combustion product components. Other factors that influence combustor design are the desires of users of gas turbine engines for efficient, low cost operation, which translates into a need for reduced fuel consumption while at the same time maintaining or even increasing engine output. As a consequence, important design criteria for aircraft gas turbine engine combustion systems include provision for high combustion temperatures, in order to provide high thermal efficiency under a variety of engine operating conditions, as well as the minimization of undesirable combustion conditions that contribute to the emission of particulates, and to the emission of undesirable gases, and to the emission of combustion products that are precursors to the formation of photochemical smog.
Various governmental regulatory bodies have established emission limits for acceptable levels of unburned hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx), which have been identified as the primary contributors to the generation of undesirable atmospheric conditions. Therefore, different combustor designs have been developed to meet those criteria. For example, one way in which the problem of minimizing the emission of undesirable gas turbine engine combustion products has been attacked is the provision of staged combustion. In that arrangement, a combustor is provided in which a first stage burner is utilized for low speed and low power conditions to more closely control the character of the combustion products. A combination of first stage and second stage burners is provided for higher power outlet conditions while attempting to maintain the combustion products within the emissions limits. It will be appreciated that balancing the operation of the first and second stage burners to allow efficient thermal operation of the engine, while simultaneously minimizing the production of undesirable combustion products, is difficult to achieve. In that regard, operating at low combustion temperatures to lower the emissions of NOx, can also result in incomplete or partially incomplete combustion, which can lead to the production of excessive amounts of HC and CO, in addition to producing lower power output and lower thermal efficiency. High combustion temperature, on the other hand, although improving thermal efficiency and lowering the amount of HC and CO, often results in a higher output of NOx.
Another way that has been proposed to minimize the production of those undesirable combustion product components is to provide for more effective intermixing of the injected fuel and the combustion air. In that regard, numerous mixer designs have been proposed over the years to improve the mixing of the fuel and air. In this way, burning occurs uniformly over the entire mixture and reduces the level of HC and CO that result from incomplete combustion. Even with improved mixing, however, higher levels of undesirable NOx are formed under high power conditions when the flame temperatures are high.
One mixer design that has been utilized is known as a twin annular premixing swirler (TAPS), which is disclosed in the following U.S. Pat. Nos. 6,354,072; 6,363,726; 6,367,262; 6,381,964; 6,389,815; 6,418,726; 6,453,660; 6,484,489; and, 6,865,889. It will be understood that the TAPS mixer assembly includes a pilot mixer which is supplied with fuel during the entire engine operating cycle and a main mixer which is supplied with fuel only during increased power conditions of the engine operating cycle. While improvements in the main mixer of the assembly during high power conditions (i.e., take-off and climb) are disclosed in patent applications having Ser. Nos. 11/188,596, 11/188,598, and 11/188,470, modification of the pilot mixer is desired to improve operability across other portions of the engine's operating envelope (i.e., idle, approach and cruise) while maintaining combustion efficiency.
In order to provide increased functionality and flexibility, the pilot mixer in a TAPS type mixer assembly has been developed and is disclosed in a patent application entitled “Pilot Mixer For Mixer Assembly Of A Gas Turbine Engine Combustor Having A Primary Fuel Injector And A Plurality Of Secondary Fuel Injection Ports.” This patent application, having Ser. No. 11/365,428, is owned by the assignee of the present application and hereby incorporated by reference. While the '428 application is concerned with the physical embodiments of the pilot mixer, it will be appreciated that an apparatus and method is desired which is able to actively control fuel flow to such pilot mixer, as well as the overall mixer assembly containing it.
It is well known that lean, premix combustion requires operation close to the lean-blow out boundary in order to minimize emissions. Therefore, it is desired that the onset of a lean blow out event be recognized so that operation of the combustor can be adjusted and lean blow out avoided. In addition, the mixing of air and fuel must be extremely effective to achieve low emissions. To enhance such mixing, pulsing the fuel to the injectors at a high frequency would also be desirable.
It has also been found that lean, premix combustion often results in high dynamic pressure levels in the combustor. The combustion dynamics is a result of interaction between heat release from combusting the fuel-air mixture and pressure oscillations in the chamber. Such dynamic pressures may result in high cycle fatigue and can damage combustor parts. While the effects of dynamic pressures on the combustor have been countered previously, this has generally involved the provision of high bandwidth fuel or air actuation to reduce the pressure levels associated with acoustic modes of the combustor.
Thus, there is a need to provide a gas turbine engine combustor in which the production of undesirable combustion product components is minimized over a wide range of engine operating conditions. Accordingly, it is desired that the pilot mixer of a nested combustor arrangement be modified to include a primary fuel injector and a plurality of secondary fuel injection ports. It is also desired that an active control system and process be provided which enhances operation of such mixer assembly by identifying and countering the onset of a lean blow out condition, as well as an unacceptable level of dynamic pressure experienced in the combustor.